Chapter 3 focuses on the politics of the winter of 1642–3, a phase of the civil war that is normally defined in terms of a “peace party” supremacy in the House of Commons and the ultimate failure of official peace negotiations between the Long Parliament and the king known as “The Treaty of Oxford.” This chapter looks instead at a quiet but crucially important diplomatic mission sent from Common Council to the king in late December 1642. The aftermath of this deputation, which unraveled from January 1643 until the late spring and included Charles I’s call for the arrest of seven leading Londoners, and in particular the civic leaders Isaac Pennington, John Venn, Randall Mainwaring, and John Fowke, radically rearticulated London’s relationship to parliament’s war effort. The political manipulation of “the attempt on the seven Londoners,” spearheaded by the accused and their allies in parliament, ushered in a period of unprecedented popular mobilization. This included the introduction of radical new propositions for an alliance between parliament and the City, the pursuit of coordinated iconoclasm, the introduction of radical metropolitan policing methods, the raising of auxiliaries, demands for new loans, and the construction of the Lines of Communication, eleven miles of fortifications built around London, Westminster, and Southwark. Chapter 3 explores how winter 1642–3 – and a previously poorly understood period of London’s wartime narrative – led to a moment of unprecedented action, a time when London’s Common Council behaved like “a third house of parliament,” a body eager to implement its radical agenda.